1. Field of the Invention
This invention generally relates to a piston mechanism of a fluid displacement apparatus, and more particularly, to a configuration of reciprocating pistons in a refrigerant compressor for use in an automotive air conditioning system.
2. Description of Related Art
A swash plate-type compressor with a variable displacement mechanism, particularly a single head, piston-type compressor suitable for use in an automotive air conditioning system, such as that described in Japanese Patent #H2-61627, which is incorporated herein by reference.
Referring to FIG. 1, the compressor, which is generally designated by reference number 100, includes a closed cylinder housing assembly formed by annular casing 1 provided with cylinder block 2 at one of its sides; a hollow portion 1a, such as crank chamber; front end plate 3; and rear end plate 26.
Front end plate 3 is mounted on one end (to the left in FIG. 1) opening of annular casing 1 to close the end opening of crank chamber 1a and is fixed on annular casing 1 by a plurality of bolts (not shown). Rear end plate 26 and a valve plate 24 are mounted on the other end of annular casing 1 by a plurality of bolts (not shown) to cover the end portion of cylinder block 2. An opening 3a is formed in front end plate 3 for receiving drive shaft 4. An annular sleeve 3b projects from the end surface of front end plate 3 and surrounds drive shaft 4 to define a shaft seal cavity 6. A shaft seal assembly 7 is mounted on drive shaft 4 within shaft seal cavity 6.
Drive shaft 4 is rotatably supported by front end plate 3 through bearing 5, which is disposed within opening 3a. The inner end of drive shaft 4 is provided with a rotor plate 8. A thrust needle bearing 14 is placed between the inner end surface of front end plate 3 and the adjacent axial end surface of rotor plate 8 to receive the thrust load that acts against rotor plate 8 and to thereby ensure smooth motion. The outer end of drive shaft 4, which extends outwardly from sleeve 3b, is driven by the engine of a vehicle through a conventional pulley arrangement (not shown). The inner end drive shaft 4 extends into center bore 2b, which is formed in the center portion of cylinder block 2 and rotatably supported therein by a bearing 15, such as a radial bearing needle bearing. The axial position of drive shaft 4 may be adjusted by means of an adjusting screw 18 which engages a threaded portion of center bore 2b. A spring device 17 is disposed between the axial end surface of drive shaft 4 and adjusting screw 18. A thrust needle bearing 16 is placed between drive shaft 4 and spring device 17 to ensure smooth rotation of drive shaft 4.
A spherical bushing 9 placed between rotor plate 8 and the inner end of cylinder block 2 is slidably mounted on drive shaft 4. Spherical bushing 9 supports a slant or swash plate 10 for nutational, (e.g., a wobbling, bobbing or nodding up-and-down motion of a spinning body as it precesses about its axis) and rotational motion. A coil spring 12 surrounds drive shaft 4 and is positioned between the end surface of rotor plate 8 and one axial end surface of spherical bushing 9 to push spherical bushing 9 toward cylinder block 2.
Swash plate 10 is connected to rotor plate 8 by a hinge coupling mechanism for rotating in unison with rotor plate 8. In particular, rotor plate 8 may have an arm portion 8a projecting outward from one side surface of rotor plate 8. In such a configuration, arm portion 10a is formed separately from swash plate 10 and is fixed on one side surface of swash plate 10.
Arm portion 8a and 10a overlap each other and are connected to one another by a pin 11 which is received by a rectangular shaped hole 8b formed through arm portion 10a of swash plate 10. In this manner, rotor plate 8 and swash plate 10 are hinged to one another. In this configuration, pin 11 is slidably disposed in rectangular shaped hole 8b, and the sliding motion of pin 11 within rectangular shaped hole 8b alters the slant angle of the inclined surface of swash plate 10.
Cylinder block 2 has a plurality of annularly arranged cylinder bores 2a into which pistons 21 slide. A cylinder arrangement may include five cylinders, but a lesser or greater number of cylinders also may be provided. Each piston 21 comprises a cylindrical body 21a slidably disposed within annularly arranged cylinder bore 2a and a connecting portion 20. Connecting portion 20 of piston 21 has a cutout portion 20b which straddles the outer periphery portion of swash plate 10. Semi-spherical thrust bearing shoes 19 are disposed between each side surface of swash plate 10 and face semi-spherical pocket 20a of connecting portion 20. Thus, swash plate 10 rotates between semi-spherical thrust bearing shoes 19, moving the inclined surface axially to the right and left, thereby reciprocating each of pistons 21 within one of annularly arranged cylinder bores 2a. Cylinder housing 1 also may include projection portion 1a extending therefrom to the inside thereof and paralleled to the reciprocating direction of piston 21.
Rear end plate 26 is shaped to define a suction chamber 27 and a discharge chamber 28. Valve plate 24, which together with rear end plate 26, is fastened to the end of cylinder block 2 by bolts (not shown), is provided with a plurality of valved suction ports 22 connected between suction chamber 27 and respective annularly arranged cylinder bores 2a, and with a plurality of valve discharge ports 23 connected between discharge chamber 28 and respective annularly arranged cylinder bores 2a. Suitable reed valves for valved suction ports 22 and valved discharge ports 28 are described in U.S. Pat. No. 4,011,029, which is incorporated herein by reference. Gaskets 25 and 29 are placed between cylinder block 2 and valve plate 24, between valve plate 24 and rear end plate 26 to seal the matching surfaces of cylinder block 2, valve plate 24, and the rear end plate 26.
As shown in the lower right hand portion of FIG. 1, crank chamber 1a and suction chamber 27 are placed in communication via a passageway 30 which comprises an aperture 30a formed through valve plate 24, and gaskets 25 and 29 and a bore 32 formed in cylinder block 2. A coupling element 31 with a small aperture 31a is disposed in the end opening of bore 32, which faces crank chamber 1a. A bellows element 34 contains gas and includes a needle valve 34a disposed in bore 32. The opening and closing of small aperture 31a, which connects between crank chamber 1a and bore 32, is controlled by needle valve 34a. The axial position of bellows element 34 is determined by a frame element 33 also disposed in bore 32. At least one hole 33a is formed through frame element 33 to permit communication between aperture 30a and bore 32.
In this configuration of a swash plate-type compressor, frictional force between swash plate 10 and spherical sleeves 19 is generated because swash plate 10 slides in spherical sleeves 19 while rotating. Thus, the frictional force acts on pistons 21 to incline them forcibly in the direction of the inner surface of cylinder bores 2a and urging them to rotate around the axis of piston 21. Further, the inner surface of cylinder bore 2a prevents piston 21 from inclining a radial direction other than to rotate. Therefore, piston 21 and cylinder bore 2a abrade each other, and piston 21 may seize against cylinder bore 2a.
In an effort to resolve this problem, the outer peripheral surface of piston 21 has been coated with a plating layer containing a self lubricating material, such as a polytetrafluoroethylene resin (hereinafter "PTFE"), so that the coated plating layer reduces friction between the periphery of piston 21 and the inner surface of cylinder bore 2a. However, this solution requires that the outer diameter of piston 21 is designed to be about 15 .mu.m to about 30 .mu.m smaller than the inner diameter of cylinder bore 2a and that a lubricating oil is introduced between piston 21 and cylinder bore 2a in order to efficiently compress a refrigerant gas.
Therefore, piston 21 and cylinder bore 2a are manufactured to precise tolerances and are assembled to closely conform to each other. As a result, the configuration is complicated to manufacture and results in a high assembling cost.
In another approach to this problem, annular piston ring 37, which is formed of a resin, such as an engineering plastic or a PTFE resin, fits into annular groove 36 formed on the periphery surface of piston 21 to seal the periphery of piston 21 and the inner surface of cylinder bore 2a without coating a plating layer on the periphery surface of piston 21. Thus, piston 21 slides in cylinder bore 2a, such that the periphery surface of piston 21 is not in direct contact with the entire inner surface of cylinder bore 2a.
In this configuration, the force, which is generated by rotation of swash plate 10 via spherical sleeves 19 and inclination of pistons 21 in the radial direction, presses annular piston ring 37 to the inner surface of cylinder bore 2a. Consequently, annular piston ring 37 may fail if no area of cylinder bore 2a is adequately secured to piston ring 37 by magnifying the width of annular piston ring 37.